1. Field of the Invention
The present invention relates to a center hole forming method and a forging device.
2. Description of Related Art
As a technique of this kind, Japanese Patent Application Publication No. 62-77144 (JP 62-77144 A) discloses a method of obtaining a first intermediate product by subjecting an object to be processed, which is inserted in a die hole, to shank extruding and then forming a center hole in both axial end surfaces of the first intermediate product without taking out the first intermediate product from the die. More specifically, a pair of second press dies is inserted in the die, so as to hold the first intermediate product from above and below. A center hole forming die is projected and provided in each of pressing surfaces of the pair of second press dies. By moving the pair of second press dies toward the first intermediate product, the center hole is formed in both axial end surfaces of the first intermediate product.
However, in the method disclosed in JP 62-77144 A, there is no description on timing to move the each second press die when the pair of second press dies is moved toward the first intermediate product. For example, if the second press die on a small diameter side of the pair of second press dies first contacts the axial end surface on the small diameter side of the first intermediate product, the first intermediate product may float in the die hole, and consequently, the center hole may not be well-formed in the axial end surface in the small diameter side of the first intermediate product.